Static reducing system



April 27, W37. F. B. M LAREN STATIC REDUCING SYSTEM Filed Dec. 19, 1952 31!: $511134 1 rriiavaall GRID vows.

H g a QZQu DEG Q .NZEESU Patented Apr. 27, 1937 UNITED PATENT OFFIE STATIC REDUCING SYSTEM Application December 19, 1932, Serial No. 647,852

8 Claims.

This invention relates to circuits and apparatus for radio receivers and more particularly to means for reducing noisy interference from lightning and other causes.

The object of this invention is to limit the value of any signal applied to a tube of a radio receiver. Another object of the invention is to attenuate all signals which exceed a predetermined value without appreciably aifecting signals which are lower than the design value. Another object is to progressively increase the attenuation as the signal becomes larger. A further object is to introduce a decrement into the tuned input circuit when signals exceed a predetermined amount. A still further object is to increase this decrement as the signal becomes larger. Other objects and features will appear from the following description.

Other attempts have been made to reduce interference from static, directional receiving antenna and circuits of extreme selectivity having been the most successful. Means which seemingly resemble this invention have been proposed in which damping of excessive signals is effected by the use of variable resistances such as crystals and vacuum tubes. In no case do these proposals go far enough and a great demand exists for further reduction of static.

Static may be thought of as extremely sudden shocks applied to an oscillating system causing waves which combine with the existing signal carrier to produce noise. In the present state of the art of receiving radio signals one or more tuned circuits are used for selecting and amplify ing the desired signal. These tuned'circuits are of low resistance and are easily set in oscillation by sudden shock as by the application of a fast rising voltage. These tuned circuits are designed to oscillate at any frequency within the desired band, usually 5,000 to 10,000 cycles per second on each side of selected frequency called the carrier. It is well known that shook excitation of a tuned circuit produces waves of all frequencies and those located in the band for which the tuned circuit is adjusted will be of appreciable value and will beat with the carrier and other signal waves to produce noise in a well known manner. The ordinary amplifying stage of a radio receiver employing vacuum tubes has a capacity for signals very much larger than necessary. This is particularly true of the first stage. Therefore, when a large static wave is impressed on the tuned circuit, it is amplified with the same ratio as a small wave of the desired signal. In fact, the tendency is to exaggerate the larger signals by the detector and produce a greater energy ratio at the output than exists at the input between wanted and unwanted signals.

In this invention, the low resistance, high efficiency tuned circuit is supplemented by a special electric discharge device which has the property of coacting with the tuned circuit in such way that signals exceeding a predetermined amount are attenuated and the output of the amplifier stage, while proportional to the signal up to the designed limit, is greatly attenuated for larger inputs. The electric discharge device consists of a gas or vapor filled tube having an electron supply from a hot cathode or an auxiliary discharge and a plate and control grid. Tubes of this character have the usually undesired characteristic of grid current, that is, the grid under normal operating conditions has a current flowing to or from it. This grid current has the property of varying in a peculiar manner with the gridcathode voltage, and advantage is taken of this property in conjunction with a special means for biasing the grid, to secure the desired attenuating characteristic.

The grid current is the resultant of two currents, one due to an electron flow from the cathode to the grid and the other due to a positive ion flow from plategrid space to grid. These currents, if the resulting electron flow is considered, are in opposite directions. The electron current will flow to the grid when the grid is less negative than the speed expressed in volts with which the electrons leave the cathode. The posi tive ion current will flow to the grid whenever such ions are present and the grid is less positive than the plate. As the normal grid potential is approximately that of the cathode, and the plate is one to two hundred volts more positive than the cathode, both currents will flow. When the grid is greatly negative to the cathode, the positive ion current predominates and current flows from the plate to the grid. A better way of thinking of this current flow will be formed if only the electron stream is mentioned. The positive ion current is neutralized by electrons pulled out of the grid and therefore this current can be spoken of as an electron current out of the grid. The density of positive ions increases when the grid is made more positive because more electrons from the cathode are allowed into the grid-plate space where ions are formed by collision. This greater ion density results in a greater current of electrons out of the grid as the grid is made more positive notwithstanding the fact that the poten-- tial diiierence between plate and grid is reduced.

The curve of current versus grid-cathode voltage resembles the plate'cathode current versus grid cathode voltage except that lower values are used. This current changes at an increasing rate until the positive ion density reaches a limit determined by the electron supply from the cathode. The electron current to the grid starts later as the grid is made less negative but increases at a faster rate than the electron current out of the grid; The resultant curve therefore starts out as an electron current out of the grid and it increases rapidly. When the electron current to the grid starts, the resultant current rate of increase slows down until when both currents are increasing at the same rate, the total current. is stationary with change of grid voltage; A further decrease of the grid potential will allow the electron current to the grid to overba lance the electron current from the grid and the, resultant quickly drops to zero. When the grid becomes still less negative, the electron current to the grid predominatesand the resultant current reverses. The important part of the grid current from the standpoint of this invention is that part where two components are changing at nearly the same rate. The point Where the two currents are changing at exactly the same rate is the ideal bias for an amplifier as then the grid-cathode has an infinite impedance and for small changes in grid-cathode voltage, no current change results; Practically, that point where the electron current from the grid is increasing faster than the electron current to the grid to a small extent, is even a better bias because the losses of the in put circuit are offset and regeneration is produced with increased amplification. Care must be taken, however, to make sure that excessive regeneration is not produced at the operating point or at any normal grid swing as oscillation may be started. To follow this action, let it be as sumed that the signal is such that at the time taken, the grid is becoming less negative, and the total current from the grid is increasing, then with normal plate return to the cathode this current has to flow through the input circuit, and as the direction of current is such that the gridmust be negative to the cathode, it will be found that this current causes the grid to become even more negative. Regeneration is thus accomplished,.as the current fiow through the input circuit is such that whatever direction the current flows it increases the voltage change which initially caused the current fiow and if the change in voltage due to this current is greater than the drop due to the circulating current, then oscillation results.

If, on the other hand, the bias point is taken at such part of the grid current curve that the electron current to the grid is increasing faster than the electron current from the grid, then the opposite result is obtained. For as the grid is made less negative, the current decreases and the drop due to grid current in the input circuit decreases and the effective grid voltage is increased or becomes more negative. If the bias point is chosen at the exact point where the two currents are changing at the same rate, then on anylarge wave, as the grid becomes less negative, an effect is produced which tends to decrease the actual grid swing, while as the grid becomesmore negative the actual grid swing increases because of regeneration. The result is a grid voltage wave which is unsymmetrical with regard to the bias point. This is similar to an actual change of bias and in practice the plate current falls as the input voltage increases.

In this invention, it is proposed to use this change in plate current to decrease the efiective grid swing and thus limit the signal. By arranging two biasing means such that one produces a fixed positive bias and the other produces a negative bias proportional to the mean plate current, the actual bias being the difference between the two, the desired result is attained. For, as the actual bias is shifted to a more positive point,

that part of the wave making the grid less negative is cut down by the anti-regenerative action of the grid current, while that part of the wave making. the grid more negative is increased to a smaller extent as the actual bias is moved in the indicated direction. The net result is that large amplitude signals are amplified to a less degree than small signals and very large signals are reduced to smaller values than normal signals.

In the drawing, Fig. 1 shows a hot cathode gas or vapor tube; Fig. 2 shows a cold cathode gas tube; Fig. 3 illustrates a connection diagram for the tubes and circuits and Fig. 4"- shows the characteristic curves of the tubes used.

In Fig. 1, the numeral l shows an indirectly heated cathode, 2 shows a control grid and 3 the plate or anode. The insulators t and 5 are used for positioning as well as shielding the three above mentioned elements. connect the control grid. to the external circuit. The glass envelope 1 and the stem 8 are sealed together and cemented to the base 9. The prongs H) are used to connect the cathode plate and heater wires tothe external circuit.

In Fig. 2, the numeral I shows the cathode of the auxiliary discharge path while II shows the anode of this auxiliary discharge which also acts as the electron source of the tube. The control grid is shown at 2 and the plate or anode at 3. The insulators 4 and 5 are used to position and shield the various elements. The top cap 6 is used to connect the control grid to the external circuit. The glass envelope 7 and stem 8 are sealed together and cemented to base 9. prongs it are used to connect the plate and elements of the auxiliary discharge to the external circuit.

In Fig. 3, the numeral I shows the indirectly heated cathode, 2 the control grid, and 3 the plate or anode. The glass envelope is shown at l and the cathode heater at 2. The input coil is designated by Q3 and the secondary or grid coil by it. The output. filter has two parts, the choke coil i5 and the condenser l 6. The condenser 57 is a bypass for the biasing means which is made up of the postive bias resistance 18 and. the negative bias resistance I9. A condenser 26 is used to tune the secondary M.

In Fig. 4', the letter A shows the grid-cathode voltage, plate current curve, while B designates the grid current grid-cathode voltage curve. The curve C shows the input conductance of the tube while D illustrates the operating bias. The line E shows the effective bias, the sine wave F illus trates the applied grid voltage while G shows the effective grid voltage. Curve I-I illustrates the grid-cathode voltage versus grid current curve for a lower plate voltage than curve B.

The tubes shown in Figs. 1 and 2 will both have grid current characteristics as shown in Fig. 4. The connections of Fig. 3 will not strictly apply to the cold cathode gas tube but the same general principle can be used. A positive potential is obtained by a tap on a resistance The top cap 6 is used to across the voltage between the plate and electron supply, while a negative potential is obtained by a resistor in series with the electron supply in the usual manner.

The invention resides in the use of the grid conductance to reduce large signals. It will be noticed that from curve C, the conductance is zero for small values of signal but has a large positive conductance on one-half of the wave and a large negative conductance on the other half for large values of signal. That part of the wave which encounters a positive conductance is lowered in value because of the drop in the input coil l4 while the other part is increased in value because of the negative conductance. It will be noticed that this phenomena causes an unsymmetrical change in the eifective grid voltage, one half wave being reduced in value While the other half is increased and if symmetry is to be had the base line must be moved to the left. This amounts to a change of bias to a more negative potential and the mean plate current is lowered. A lower plate current will produce a smaller drop in resistance l9 and the bias from resistance I8 becomes of greater importance and the applied bias becomes less negative. At the new bias, the drop in value of the half wave which encounters the positive con ductance becomes more pronounced while the amplification of the other part becomes less. This results in a still further change in bias and a greater drop in value of the effective grid swing. By arranging the resistances l8 and I9 so that a large positive bias is opposed by a large negative bias, then a small increase in value of signal will produce a large decrease in effective value of grid swing. In fact, I have built tubes and biasing means such that doubling of the signal reduced the output to half.

The particular biasing means shown is not essential to the operation of the invention, as the ordinary means of a series resistor in the cathode circuit or a special C battery may be used with good results. The means illustrated 7 will tend to increase the effect of the variable grid conductance and is the preferred method.

The curve H in Figure 4 shows the effect of lower plate voltage and therefore shows the efiect of a load in the plate circuit. A load such as the primary of the next stage transformer or any other coupling means will tend to increase the steepness of the right hand portion of the grid current curve while flattening the left hand portion. The damping or lowering in value of the half wave is increased but the regeneration on the other half wave is also decreased, if the load is a resistance. For tuned circuits or a resistance load in the plate circuit the damping is increased and greater static reduction is obtained but the special biasing means is of less importance.

What I claim is:

1. In a radio receiving system, means for reducing noise which comprises a tuned circuit of high efficiency coupled to a receiving antenna and to a vapor filled hot cathode tube, the tube grid being biased at such point that for small signals the input conductance is negative and for large signals the input conductance is positive to the end that oscillations produced by impulse type static are quickly reduced to a low value, and a load circuit for coupling this network to the remainder of the receiver.

2. In a radio receiving system, means for reducing static which comprises an antenna coupled to a gas filled cold cathode tube whose variable input conductance is shunted across the tuned circuit, biasing means whereby the average grid-cathode voltage may be set at that value where small signals in the tuned circuit are met with a negative conductance in the tube circuit shunted across this tuned circuit while large signals are met with a positive conductance wl'L'ch damp oscillations due to the impact of static, a load circuit which couples the tube to the next stage and a source of heater, grid and plate energy.

3. In a network for reducing static including a gas filled tube having a variable input conductance, biasing means for positioning the grid voltage such that a signal exceeding the desired limit will lower the grid current at a high rate on one half of the wave but will decrease it at a lower rate on the other half.

4. The method of reducing interference in a radio receiving system having a tuned first stage with an electric discharge device having a variable input conductance which consists in utilizi ing a negative input conductance for small signals and a positive or damping conductance for large signals.

5. The method of reducing static in a radio receiving system having a discharge tube with an input conductance varying from a negative value to a positive value which consists in using the negative input conductance to aid in amplification of part of the wave and the positive input conductance to damp or reduce a part of the wave.

6. The method of reducing static in a radio receiving system having a tuned circuit shunted by a conductance varying from negative to positive, which consists in applying the signal to the tuned circuit and using the variable conductance to highly damp one half of the signal Wave, that part of said wave which exceeds the design limit being damped to a greater extent.

'7. The method of reducing static in a radio receiving system having a tuned circuit shunted by a vapor filled amplifying tube biased at such point that it has a variable input conductance to alternating voltage, being negative for half the wave and positive for the other half, which consists in utilizing the positive conductance to damp oscillation in the tuned stage caused by static.

8. In a radio receiving system means for damping excessive oscillations, said means comprising a tuned oscillating circuit and a gasfilled three-element tube connected to said circuit, said tube having a grid bias bringing signal impulses on the straight section of the platecurrent curve and adjacent the point of reversal of the grid-current curve.

FRED. B. MACLAREN.

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